I've been keeping observations like this since 1991 and its a really difficult thing to do! You would think its a definite moment when something flowers but in reality its hard to decide on the moment.

Another thing is that the observations should be of the SAME clump/tree/plant/ every year. Other plants in different locations will not come out at the same time. I have a great book, which I found in a 2nd hand bookshop. It was written in the first half of the 20C. There are photos of the SAME clump of snowdrops on two dates for each year from about 1908 to 1945 (from memory). Similar photos for a clump of daffs and a branch of horse chestnut. The variations in development are enormous. (The 1940s were aweful!) If I have time I might scan a few for you - I think it will be out of copyright now.

I would say that Anne as nailed it re flowers/flauna, although it still warrants a mention when your first clump of daffs/snowdrops, etc flower for eg: your garden, churchyard and obviously they should be in the ground, not pots.
It's a bit like defining oktars or % cloud cover and what cloud type exactly - a little awkward when determining a 'flower date'.

dates of first bee, swallow, house martin, etc are straight forward.
dates of birds (type) nesting and fledging are a little more circumspect - it's not just when you see them enter a box, but taking in their materials - nest building (as opposed to a box) is again more straightforward.

this time last year I alreday had swallows nesting in a resident house martins nest - no sign yet.